Zinc-related actions of sublethal levels of benzalkonium chloride: Potentiation of benzalkonium cytotoxicity by zinc

Abstract Benzalkonium chloride (BZK) is a common preservative used in pharmaceutical and personal care products. ZnCl 2 was recently reported to significantly potentiate the cytotoxicity of some biocidal compounds. In the present study, therefore, we compared the cytotoxic potency of BZK and then further studied the Zn 2+ -related actions of the most cytotoxic agent among BZK, using flow cytometric techniques with appropriate fluorescent probes in rat thymocytes. Cytotoxicity of benzylcetyldimethylammonium (BZK-C16) was more potent that those of benzyldodecyldimethylammonium and benzyldimethyltetradecylammonium. ZnCl 2 (1–10 μM) significantly potentiated the cytotoxicity of BZK-C16 at a sublethal concentration (1 μM). The co-treatment of cells with 3 μM ZnCl 2 and 1 μM BZK-C16 increased the population of both living cells with phosphatidylserine exposed on membrane surfaces and dead cells. BZK-C16 at 0.3–1.0 μM elevated intracellular Zn 2+ levels by increasing Zn 2+ influx, and augmented the cytotoxicity of 100 μM H 2 O 2 . Zn 2+ is concluded to facilitate the toxicity of BZK. We suggest that the toxicity of BZK is determined after taking extracellular (plasma) and/or environmental Zn 2+ levels into account.